This invention relates to a method for determining the air-tightness of enclosed spaces. In particular, the invention relates to a method whereby, for permanently inertizable spaces inertized to prevent and/or extinguish a fire, the corresponding volume-based leakage rate can be determined with a maximum degree of accuracy.
The document FR 2 834 066 A1 describes a leakage detection method employing oxygen/gas sensors. The prior-art measuring principle is based on the fact that the partial gas pressure component on the detector element is modified by the inward or outward seepage of an inert or reactive test gas.
The document DE 102 51 536 A1 describes a method for minimizing gas consumption in gas-filling operations and for leak detection in such gas-filling processes. That prior-art method employs a test gas serving to eliminate the need for replenishment.
The document JP 63 214635 A describes yet another leak detection method whereby a test gas is introduced in the atmosphere of an enclosed container. The object to be tested (for its gas tightness) is placed in a container with a gas detector built into said object. It can then be determined whether the test gas penetrates into the object by permeating the walls of the object.
Inertization procedures for lessening the risk of a fire in an enclosed space have been known from fire fighting technology. These inertization procedures typically involve the injection of an oxygen-displacing gas from an inert-gas source so as to lower the air atmosphere in the enclosed space to, and maintain it at, an inert level below the oxygen concentration in the ambient air atmosphere. The preventive and extinctive effect of that process is based on the principle of oxygen depletion. Normal ambient air is known to consist of about 21% by volume oxygen, 78% by volume nitrogen and 1% by volume other gases. To reduce the risk of a fire breaking out and/or to extinguish a fire that has already broken out in an enclosed space, the introduction for instance of pure nitrogen as the inert gas further increases the nitrogen concentration in the enclosed space concerned and reduces the proportional oxygen content. An extinctive effect is known to set in when the oxygen component drops off to below about 15% by volume. Depending on whatever flammable materials are present in the enclosed space, the oxygen component may have to be further reduced to perhaps 12% by volume. Most flammable materials cannot burn at that oxygen level.
When in an ancillary inert-gas fire extinguishing system employing the inert-gas fire fighting technology referred to above the highest possible safety standard is to be met, it will be necessary to provide for facility- and logistics-related planning in the event of an operational shut-down due to functional failures, in order to comply with the established safety requirements. Yet even if in designing the inert-gas fire fighting system all measures are taken into account that permit the quickest and smoothest possible resumption of the operation, the inertization of enclosed spaces nevertheless entails certain problems and is clearly limited in terms of fail-safe operation. It has been found that, while it is possible to design a fire extinguishing system in a way as to make a failure during the lowering or adjustment of the oxygen content in the enclosed space to an inert level relatively unlikely, it is often difficult to maintain that lowered, inert state at the required level for an extended period, especially for the duration of the so-called “emergency operation phase”. This is due primarily to the fact that prior-art inertization methods do not offer the possibility of preventing the flashback threshold of the oxygen concentration in the enclosed space from being prematurely exceeded when a disruption causes all or at least part of the inert-gas feed to fail.
The above-mentioned flashback stage is defined by the time segment following the so-called “fire fighting phase” during which the oxygen concentration in the enclosed space must not exceed a specific value, the so-called “flashback prevention threshold”, to avoid reignition of the materials present in the protected area. The flashback prevention threshold is an oxygen concentration that depends on the fire load of the enclosed space and is determined by experimentation. According to industrial safety regulations, the oxygen concentration in the enclosed space, when flooded, must be such that the flashback prevention threshold of for instance 13.8% by volume is not reached within the first 60 seconds after the flooding began. These 60 seconds after the start of the flooding are also known as the “fire fighting phase”.
Nor must the flashback prevention threshold be exceeded within 10 minutes after the end of the fire fighting phase. This is based on the premise that within the fire fighting phase the fire in the protected area is fully extinguished. The time segment (for instance 10 minutes) following the fire fighting phase, intended to make certain that the fire extinguished-during the fire fighting phase will not flare up again, is referred to as the “flashback stage”.
In applying prior-art inertization methods it is customary, immediately upon detection of a fire in the enclosed space, to reduce the oxygen concentration in the atmosphere of the enclosed space as quickly as possible to a so-called “operating concentration”. The inert gas required to that effect is usually supplied by an appropriate inert-gas source that is part of the inert-gas fire extinguishing system. The term “operating concentration” or “operating concentration level” refers to an inert state below a so-called “configurational concentration” for the specific enclosed space concerned.
The “configurational concentration” of the enclosed space concerned is an oxygen concentration in the atmosphere of the enclosed space at which the ignition of any material present in the enclosed space is effectively prevented. In other words, the “configurational concentration level” in the enclosed space concerned represents the inertization level at which the ignition of any materials present in the enclosed space is effectively prevented. When setting the configurational concentration, i.e. the configurational concentration level for an enclosed space, a further safety margin is usually added below the threshold, i.e. deducted from the “concentration threshold value” at which no ignition of any material in the enclosed space can take place.
Once the operating concentration has been reached in the internal air atmosphere of the enclosed space, the oxygen concentration is usually maintained, by means of a control concentration setting below the operating concentration of the enclosed space, at a so-called “control concentration level”. This control concentration is a control range of the residual oxygen concentration in the inertized internal air atmosphere of the enclosed space within which the oxygen concentration is maintained during the flashback stage. That control range is usually delineated by an upper limit that defines the threshold for activating the inert-gas source, and a lower limit that defines the threshold for deactivating the inert-gas source of the inert-gas fire extinguishing system. During the flashback stage the control concentration is usually maintained within that control range by the repeated injection of inert gas. As stated above, the necessary inert gas is usually supplied by the inert-gas source of the inert-gas fire extinguishing system in the form of a reservoir, i.e. a device serving to generate an oxygen-displacing gas (such as a nitrogen generator), or from gas bottles or some other buffer supply unit.
However, a danger in the event of a malfunction or disruption of the inert-gas fire extinguishing system consists in the possibility of a premature increase of the oxygen concentration in the internal air atmosphere of the enclosed space, thus exceeding the flashback prevention threshold before expiration of the above-mentioned 10 minutes after the end of the fire fighting phase, i.e. before the end of the flashback stage. That would shorten the flashback stage and under certain circumstances it may no longer be possible to ensure a successful suppression of the fire in the enclosed space.
Addressing the above-described problem with regard to the industrial safety requirements for an inert-gas fire extinguishing system, i.e. for an inertization method, EP 1 550 481 A1 introduces an inertization method whereby the oxygen content in the internal air atmosphere in the enclosed space is reduced to a control concentration at a level below the operating concentration in that space, with both the control concentration and the operating concentration, along with a fail-safe margin, reduced far enough below the configurational concentration established for the enclosed space to cause the upslope of the oxygen content in the internal air atmosphere of the enclosed space, in the event of a malfunction of the inert-gas source, to reach a concentration threshold value determined for the enclosed space only after a predefined time interval. In particular, that concentration threshold value is the flashback prevention threshold for the enclosed space.
The flashback prevention threshold corresponds to an oxygen concentration in the internal air atmosphere of the enclosed space at which flammable materials in the enclosed space are certain not to be ignitable anymore. Expressed in other words, the prior-art solution referred to provides for the operating concentration to be set so low from the start that the upslope of the oxygen concentration will not reach the concentration threshold value until after a particular time, that time being long enough to initiate a flashback stage during which the oxygen content does not exceed the flashback prevention threshold, thus effectively preventing an ignition or reignition of flammable materials in the enclosed space.
This so-called “ramp-down” of the operating concentration, i.e. setting the operating concentration along with an additional fail-safe margin below the configurational concentration level of the enclosed space, makes it possible in the event of a breakdown of the inert-gas source to maintain the oxygen concentration below the flashback prevention threshold at least for the duration of an emergency operation.
The size of the additional fail-safe margin, i.e. the question of the extent to which the operating concentration must be set below the configurational concentration of the enclosed space, depends most of all on the air exchange rate to which the enclosed space is exposed. In inert-gas fire fighting technology, n50 is the value serving as the primary measure for determining the air tightness of an enclosed space.
The n50 air exchange rate is a function of the air flow volume per hour when a differential pressure of 50 Pa is maintained, divided by the volume of the structure. Accordingly, the lower the air exchange rate, the higher the air-tightness rating.
The n50 value as an indicator of the air tightness of an enclosed space is usually measured by a differential-pressure (Blower-Door) method. In the case especially of larger buildings or rooms, however, conducting a differential pressure test series for determining the n50 air exchange rate is often possible only under certain difficult conditions since establishing a pressure difference of 50 Pa between the internal air atmosphere in the enclosed space and the ambient air atmosphere outside the enclosed space is often found to be unattainable. Moreover, when a differential-pressure measurement is conducted, one cannot rule out the possibility of a change in the atmospheric condition within the enclosed space during the course of the test especially in terms of the air exchange rate. For example, the positive and negative pressures necessarily applied in the enclosed space during the differential-pressure measuring process may conceivably cause originally sealed openings to leak. Even the contents of the enclosed space, such as objects or merchandise (especially in the case of a storage facility) will affect the n50 air exchange rate determined by the differential-pressure measurement.
Since the air exchange rate of the enclosed space can only be measured with a certain degree of unreliability, if at all, it is necessary in the above-mentioned inertization process to make the additional fail-safe margin sufficiently large in order to meet the industrial safety requirements. Yet providing such a large safety margin has an unfavorable impact on the routine operating cost of the inert-gas fire extinguishing system concerned since it always involves the injection of substantially more inert gas into the enclosed space than would actually be necessary.